Environmental monitoring is continuing Sunday after an explosion and series of fires at Husky Energy’s oil refinery caused a temporary evacuation last Thursday.

Officials with the Wisconsin Department of Natural Resources and U.S. Environmental Protection Agency and the company continue to monitor air and water quality around the refinery. So far, there appear to be no health concerns associated with the incident. As the community recovers, questions are mounting over the refinery’s use of hydrogen fluoride, a highly toxic chemical.

The hazardous chemical could have posed devastating health impacts for the port community of 27,000. However, Superior Fire Chief Steve Panger said the tank with hydrogen fluoride had special water systems around it that were immediately activated when the explosion occurred.

“Obviously, once we had a fire, we were concentrating on that area to make sure that didn’t get compromised,” said Panger.

At 15,000 pounds, the tank had less than the roughly 75,000 pounds of hydrogen fluoride it usually contains. Panger said they used that information along with plume models and weather patterns to determine the evacuation area. At the same time, he said the tank was being constantly cooled as crews tried to control the perimeter of the fire, which was about 150 feet away.

“The hydrogen fluoride tank, which has not been compromised, has a dedicated fire suppression system, and will be part of the full investigation,” said Husky Energy spokeswoman Kim Guttormson in an email Sunday.

She added that an initial investigation is underway in areas of the refinery that are safe to access. She also noted that workers will continue to be paid as cleanup and repairs begin.

The use of hydrogen fluoride makes Superior’s refinery one of the 50 most dangerous refineries in the country, according to Fred Millar, an independent consultant on chemical risks with such facilities based in Washington, D.C.

“The reason is using hydrogen fluoride instead of a non-disaster chemical alternative, which is sulfuric acid,” said Millar. “Sulfuric acid will not put a big, huge toxic gas cloud into your community.”

A 2011 report from the Center for Public Integrity detailed that hydrogen fluoride is used among 50 out of 148 refineries nationwide. The chemical is highly toxic, according to Ron Koopman, a retired scientist with the Lawrence Livermore National Laboratory in California.

“It destroys tissue in horrible ways,” he said. “If you get it on your skin, it works its way toward the bone destroying all of the tissue in between.”

Hydrogen fluoride is used in refineries as part of the alkylation process, and it’s used to make higher octane gasoline. Koopman said it’s an acid that attacks mucous membranes and can cause lung damage when people are exposed to it, whereas sulfuric acid doesn’t have the potential to become airborne if released. However, Millar said it’s a big deal for refineries to switch from hydrogen fluoride to sulfuric acid.

The Center for Public Integrity puts that cost somewhere between $50 million and $150 million for each refinery.

Superior Mayor Jim Paine said the refinery’s use of hydrogen fluoride will be part of conversations going forward.

“There are refineries that don’t use hydrofluoric acid for the refinement of gasoline,” said Paine. “I think we do need to look at other options, but that’s something we’re going to be working on with the company going forward. We’re still in a public safety and cleanup phase.”

Around 20 people were treated for injuries in Thursday’s incident, but all have either been released or are reported to be in good condition.

The United Steelworkers has urged refineries across the country to phase out the use of hydrofluoric acid, saying more than 26 million people are at risk in the event of a release.

Around 20 people were treated for injuries in Thursday’s incident, but all have either been released or are reported to be in good condition.